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SERIAL DEVICE NAMING

    As of 2.6.10, serial devices on ia64 are named based on the

    order of ACPI and PCI enumeration.  The first device in the

    ACPI namespace (if any) becomes /dev/ttyS0, the second becomes

    /dev/ttyS1, etc., and PCI devices are named sequentially

    starting after the ACPI devices.

    Prior to 2.6.10, there were confusing exceptions to this:

        - Firmware on some machines (mostly from HP) provides an HCDP

          table[1] that tells the kernel about devices that can be used

          as a serial console.  If the user specified "console=ttyS0"

          or the EFI ConOut path contained only UART devices, the

          kernel registered the device described by the HCDP as

          /dev/ttyS0.

        - If there was no HCDP, we assumed there were UARTs at the

          legacy COM port addresses (I/O ports 0x3f8 and 0x2f8), so

          the kernel registered those as /dev/ttyS0 and /dev/ttyS1.

    Any additional ACPI or PCI devices were registered sequentially

    after /dev/ttyS0 as they were discovered.

    With an HCDP, device names changed depending on EFI configuration

    and "console=" arguments.  Without an HCDP, device names didn't

    change, but we registered devices that might not really exist.

    For example, an HP rx1600 with a single built-in serial port

    (described in the ACPI namespace) plus an MP[2] (a PCI device) has

    these ports:

                                  pre-2.6.10      pre-2.6.10

                    MMIO         (EFI console    (EFI console

                   address        on builtin)     on MP port)    2.6.10

                  ==========      ==========      ==========     ======

      builtin     0xff5e0000        ttyS0           ttyS1         ttyS0

      MP UPS      0xf8031000        ttyS1           ttyS2         ttyS1

      MP Console  0xf8030000        ttyS2           ttyS0         ttyS2

      MP 2        0xf8030010        ttyS3           ttyS3         ttyS3

      MP 3        0xf8030038        ttyS4           ttyS4         ttyS4

CONSOLE SELECTION

    EFI knows what your console devices are, but it doesn't tell the

    kernel quite enough to actually locate them.  The DIG64 HCDP

    table[1] does tell the kernel where potential serial console

    devices are, but not all firmware supplies it.  Also, EFI supports

    multiple simultaneous consoles and doesn't tell the kernel which

    should be the "primary" one.

    So how do you tell Linux which console device to use?

        - If your firmware supplies the HCDP, it is simplest to

          configure EFI with a single device (either a UART or a VGA

          card) as the console.  Then you don't need to tell Linux

          anything; the kernel will automatically use the EFI console.

          (This works only in 2.6.6 or later; prior to that you had

          to specify "console=ttyS0" to get a serial console.)

        - Without an HCDP, Linux defaults to a VGA console unless you

          specify a "console=" argument.

    NOTE: Don't assume that a serial console device will be /dev/ttyS0.

    It might be ttyS1, ttyS2, etc.  Make sure you have the appropriate

    entries in /etc/inittab (for getty) and /etc/securetty (to allow

    root login).

EARLY SERIAL CONSOLE

    The kernel can't start using a serial console until it knows where

    the device lives.  Normally this happens when the driver enumerates

    all the serial devices, which can happen a minute or more after the

    kernel starts booting.

    2.6.10 and later kernels have an "early uart" driver that works

    very early in the boot process.  The kernel will automatically use

    this if the user supplies an argument like "console=uart,io,0x3f8",

    or if the EFI console path contains only a UART device and the

    firmware supplies an HCDP.

TROUBLESHOOTING SERIAL CONSOLE PROBLEMS

    No kernel output after elilo prints "Uncompressing Linux... done":

        - You specified "console=ttyS0" but Linux changed the device

          to which ttyS0 refers.  Configure exactly one EFI console

          device[3] and remove the "console=" option.

        - The EFI console path contains both a VGA device and a UART.

          EFI and elilo use both, but Linux defaults to VGA.  Remove

          the VGA device from the EFI console path[3].

        - Multiple UARTs selected as EFI console devices.  EFI and

          elilo use all selected devices, but Linux uses only one.

          Make sure only one UART is selected in the EFI console

          path[3].

        - You're connected to an HP MP port[2] but have a non-MP UART

          selected as EFI console device.  EFI uses the MP as a

          console device even when it isn't explicitly selected.

          Either move the console cable to the non-MP UART, or change

          the EFI console path[3] to the MP UART.

    Long pause (60+ seconds) between "Uncompressing Linux... done" and

    start of kernel output:

        - No early console because you used "console=ttyS".  Remove

          the "console=" option if your firmware supplies an HCDP.

        - If you don't have an HCDP, the kernel doesn't know where

          your console lives until the driver discovers serial

          devices.  Use "console=uart, io,0x3f8" (or appropriate

          address for your machine).

    Kernel and init script output works fine, but no "login:" prompt:

        - Add getty entry to /etc/inittab for console tty.  Look for

          the "Adding console on ttyS" message that tells you which

          device is the console.

    "login:" prompt, but can't login as root:

        - Add entry to /etc/securetty for console tty.

    No ACPI serial devices found in 2.6.17 or later:

        - Turn on CONFIG_PNP and CONFIG_PNPACPI.  Prior to 2.6.17, ACPI

          serial devices were discovered by 8250_acpi.  In 2.6.17,

          8250_acpi was replaced by the combination of 8250_pnp and

          CONFIG_PNPACPI.

[1] http://www.dig64.org/specifications/DIG64_PCDPv20.pdf

    The table was originally defined as the "HCDP" for "Headless

    Console/Debug Port."  The current version is the "PCDP" for

    "Primary Console and Debug Port Devices."

[2] The HP MP (management processor) is a PCI device that provides

    several UARTs.  One of the UARTs is often used as a console; the

    EFI Boot Manager identifies it as "Acpi(HWP0002,700)/Pci(...)/Uart".

    The external connection is usually a 25-pin connector, and a

    special dongle converts that to three 9-pin connectors, one of

    which is labelled "Console."

[3] EFI console devices are configured using the EFI Boot Manager

    "Boot option maintenance" menu.  You may have to interrupt the

    boot sequence to use this menu, and you will have to reset the

    box after changing console configuration.